In the production of nuclear grade uranium hexafluoride, crude UF6 containing small amounts of vanadium, titanium, and to a greater extent, molybdenum are vaporized and fed into a boiler still for processing. While the low boiling impurities, i.e. HF, VOF3 and MoF6, are easily removed before the distillation of UF6 commences, the molybdenum contamination which exists as molybdenum oxytetrafluoride, MoOF4, remains. Continuous batch-to-batch refinement of crude uranium hexafluoride results in the accumulation of molybdenum impurity in the high boiler still bottom. At some point, co-distillation of the molybdenum impurity with the UF6 occurs, and the UF6 composition containing the MoOF4 fails to meet regulatory specifications. This molybdenum-containing UF6 composition must then be disposed, or stored until a suitable recovery technology is developed. Capturing the uranium values and/or the molybdenum values would enhance the economics of the UF6 production process.
Conventional separation methods for recovering uranium values from acid leach liquors are known. Generally, these processes utilize either ion exchange or solvent extraction technology. However, these methods are difficult to implement in the presence of high molybdenum and fluoride ion concentrations, as would be the case when using such methods to treat the molybdenum-containing UF6 composition.
The prior art has recognized a need for removing molybdenum from uranium. However, prior art methods have not addressed the problem of molybdenum recovery. Such methods include the ion-exchange work of Fox et al. in U.S. Pat. No. 3,790,658; Ruiz et al. U.S. Pat. No. 4,092,399 or Kuehl et al. in U.S. Pat. No. 4,304,757. The limitations of these methods for reclamation of metal values to treat the molybdenum-containing UF6 composition are pronounced. For example, it is well known that molybdenum will react similarly to uranium in forming anionic complexes which will be adsorbed on resins. As the molybdenum values continue to increase on the resin, a decrease in the total uranium capacity results. In practice, efforts are made to control the molybdenum concentration by blending ore leachates to keep the levels of molybdenum in the range of 0.01 to 0.02 grams per liter. However, in dealing with composition of the molybdenum-containing UF6 composition addressed by the present invention, concentrations of up to 90% by weight of molybdenum or higher may be present. Clearly, dilution to achieve separation by these methods is not economical.
Of much greater concern is the high fluoride ion concentration produced during hydrolysis of the process heavies for this system. It has been found that premature breakthrough of uranium and molybdenum occurs when high concentrations of fluoride ion (>=1000 ppm) is present. This inevitably results in an unacceptably high cross contamination factor for the reprocessed metals as well as a final product which contains fluoride ion exceeding the tolerance limits. An alternative process is needed.
Attempts to use conventional solvent extraction technology such as that described in U.S. Pat. No. 4,011,296 results in limited practicality. High concentrations of molybdenum in the process waste build up in the amine extractant and act as a “poison” in a manner similar to that observed with the ion exchange resins. This usually occurs when the concentration of molybdenum exceeds 0.03 g per liter of organic phase. Eventually, a maximum tolerance level is reached after which point precipitation of complex amine heteropolymolybdates occurs. The precipitate forms at the organic-aqueous interface as a gummy mass which seriously interferes with the operation. Compounding the problem is the high fluoride ion concentration present when the molybdenum-containing UF6 composition is treated by such a process. The formation of uranyl fluorides is possible, thus causing these complexes to be retained in the organic phase during stripping. The result is an unacceptable cross-contamination level in the recovered metals. The high fluoride ion concentration also augments the problem by interfering with a rapid phase separation.
An attempt was made to use the precipitation technique disclosed by Crossley in U.S. Pat. No. 4,393,028; however, the technique was found to be incompatible for use with the molybdenum-containing UF6 composition. Undesirable cations deleterious to the recycling of the uranium values are introduced and all products become cross contaminated with fluoride containing compounds to an extent which prevents recycling or resale of the product.
Even a previous patented process for recovering these metal values, see U.S. Pat. No. 4,584,184, which is incorporated herein by reference, requires the use of large quantities of organic alcohols, which due to changing environmental regulations is no longer a desirable process. It is therefore apparent, that a method to recover the molybdenum and uranium values from molybdenum-containing UF6 compositions avoiding the use of any organic substrates would be most desirable.